1. Field of the Invention
This invention relates to the calibration of analog circuitry in a monopulse receiver and more particularly to a digital correction circuit for compensating drifts and anomalies in phase and gain between quadrature channels.
2. Description of the Prior Art
A number of airports in the United States have parallel runways for the landing and take off of airplanes. When the weather is inclement so that the runways may not be visually observed by the pilot, one runway is shut down allowing only one runway to be used during periods of inclement weather.
A precision runway monitor may be provided to receive RF signals radiated by airplanes in response to a ground based air traffic control station. A monopulse radar receiver positioned to scan the area of airplanes flying an approach pattern to land on one or the other runway may be tracked from the signals radiated by the respective airplane. A requirement was established that the monopulse radar receiver should have an azimuth accuracy of 60 feet at a range of 60,000 feet which corresponds to 0.057.degree. accuracy. By using the resulting resolution, aircraft approaching the runway may be prevented from colliding into one another and assured of a landing on the correct runway. For example, the runways may be parallel to one another and spaced apart by 3,500 feet. The precision runway monitor may be placed on the ground midway between the two runways.
A typical monopulse receiver would have an antenna which simultaneously generates a sum and difference signal-in response to a received signal emanated by an aircraft. In order to process all the energy that is received indicative of location, the sum signal and difference signal are each processed by in phase and quadrature channels. It is well known that different gain and phase variations in the in-phase and quadrature channels result in an associated error in the sum signal and the difference signal which ultimately results in an error in the azimuth measurements of the incoming signals.
In a publication by F. E. Churchill et al. entitled "The Correction of I and Q Errors In A Coherent Processor", IEEE Transactions on Aerospace and Electronic Systems, Vol. AES-17, No. 1, January, 1981, pp. 131-137, a method is presented for correcting the I and Q channel errors by means of correction coefficients which are derived from measurements of a test signal. The required correction coefficients can be obtained from time samples of a test signal. By taking the discrete Fourier transformation (DFT) of the samples, three-digital filters are formed at dc, the test signal frequency, and the image of the test signal frequency. By using a simple algorithm, errors in gain may be inserted into one channel and errors in phase may be inserted into the other channel.
In U.S. Pat. No. 4,040,055 which issued on Aug. 2, 1977 to T. H. Donahue et al. entitled "Digital Compensator For Transmitter Instability", the in-phase and quadrature coherent phase detector outputs are continuously monitored for phase, amplitude and timing instabilities and arithmetic correction signals are generated and applied to the digitized received signal values to compensate for these instabilities.
In U.S. Pat. No. 4,021,804 which issued on May 3, 1977 to Dounce et al., the use of digital techniques is described for storage and for phase correction processing of the received signals in a coherent-on-receive pulse radar system. The phases of the received video signals are corrected by first converting the video signals to a series of complex digital numbers indicating the amplitude and phase of the received signals with respect to the stable local oscillator. The stored complex digital numbers are then operated on by a digital data processor to provide a new series of digitally corrected complex numbers representing coherent received video signals.
In a publication by A. I. Sinsky and P. C. P. Wang, entitled "Error Analysis of a Quadrature Coherent Detector Processor", IEEE Transactions On Aerospace and Electronic Systems, Vol. AES-10, No. 6, November, 1974, pp. 880-883, the output signal distortion resulting from relative phase and gain errors between two quadrature detector channels of a baseband processing system is computed.